The present invention is related to electronic circuits which drive signals onto transmission lines and, more particularly, to CMOS circuits which drive differential digital signals onto transmission lines.
In the transmission of electrical signals, the signals are driven onto the transmission lines at one terminal for reception at a second, remote terminal. Differential signals are used in which signals are transmitted over a pair of lines. The difference in the voltages on the two lines represents the transmitted signal at the receiving terminal. Most electrical noise is not a problem since the interfering noise is cancelled out by the differential nature of the signals.
However, a recurring problem in electrical signal transmission is the possibility that the reference voltage, i.e., ground, may not be the same at the two terminals. This offset of ground may cause a latchup condition in the circuits at the terminal at the higher ground potential. To avoid this problem, digital signal driver circuits, such as NTL (NMOS Transceiver Logic) and GTL (Gunning Transceiver Logic), offset the range of switching voltages from ground.
Nonetheless, these digital signal driver circuits have a limited offset. If the difference in the ground reference exceeds 1 volt, latch-up conditions will still occur.
To avoid these problems the present invention provides for an effective, yet inexpensive, solution for driving digital differential signals using CMOS circuits, even though a high offset ground at one terminal of a transmission line.